In devices and systems incorporating photonic chips, it is often necessary to couple light from an on-chip waveguide into an off-chip optical fiber. With reference to FIG. 1, in a conventional edge coupling arrangement, a photonic chip 110 includes a waveguide 120, such as an edge coupler, that directs light towards a facet 130 of the photonic chip 110. The light is coupled out of the facet 130 into an off-chip optical fiber 140. Typically, owing to fabrication process limitations, the facet 130 is not perfectly perpendicular to the propagation path of the light and the plane of the photonic chip 110, which can reduce coupling efficiency. Accordingly, it is desirable to determine the facet angle θ, defined relative to the plane of the photonic chip 110 in FIG. 1.
Conventionally, scanning electron microscopic (SEM) techniques are used to determine the facet angle. In one technique, the facet angle is determined from top-view or perspective-view SEM images of chips on a wafer. Unfortunately, the accuracy and throughput of this technique may be inadequate, and the number of measurement sites that can be accommodated by the technique may be limited. In another technique, the facet angle is determined from cross-sectional SEM images of chips on a wafer. Although this technique provides a higher accuracy, the technique often requires additional and/or destructive processing steps.
Accordingly, there is a need for new methods, test structures, and test systems for determining the facet angle and other surface characteristics of a chip facet, particularly at the wafer scale.